Silica species quartz

To each of these systems a measured amount (s 5g) of crushed (< 125p) quartz was added in a stainless-steel beaker, and the systems were heated to their boiling points at atmospheric pressure for 5 hr. Quartz was chosen for these experiments because of its tight crystalline structure and relative insolubility in relation to other silica species (2). Solubility values for this mineral would represent a minimum for a given solvent system with respect to silica. 

Opal is related to the very common Si02 mineral species, quartz. Oceans are at present undersaturated with respect to opal (Broecker, 1971) possibly because of the biological formation of animals with silicified skeletons such as the diatoms. These delicate structured creatures, which proliferate in the upper photic zone, dissolve at depth. Therefore, only robust siliceous skeletons such as sponge spicules are retained in sediments that accumulate in deep waters, although some diatoms survive on the continental shelf under zones with high productivity. The initial deposition of the amorphous hydrated silica, opal, converts first to opal-CT and eventually to crystalline quartz (Kastner, 1981). 

Silica precipitation is controlled by the concentration of silica in solution, itself related to silica availability and, to a lesser extent, the duration of wetting/drying cycles (Knauth, 1994). The most soluble silica species will be precipitated first from any supersaturated solution (Millot, 1960, 1970). This is usually amorphous silica, which is both poorly ordered and has a higher solubility under neutral conditions (60-130ppm) compared with opal-C (20-30ppm) and quartz (6-10ppm) (Williams et al., 1985). Silica monomers polymerise and aggregate to form colloids from solutions, and these colloids may precipitate to form opal-A (Iler, 1979 Williams et al., 1985). If solutions are supersaturated with respect to quartz then... 

AQUEOUS SILICA SPECIES AND THE SOLUBILITIES OF QUARTZ, AMORPHOUS SILICA, AND OTHER SILICA POLYMORPHS... 

Richer et al. (1982) present thermodynamic data for silica species equivalent to a quartz solubility of 6 ppm at 25°C, which corresponds to a concentration of 1 O " mol/kg. The same solubility is supported by Fournier, who gives log (quartz) = 0.41 - 1309/T(K) (see Nordstrom et al. 1990). We will assume a quartz solubility value of 6 ppm at 25°C in future calculations and discussions. 

Figure 7.7 Solubilities of quartz and amorphous silica as a function of pH. Also shown are lines indicating the solubility of quartz due to the individual dissolved silica species H4Si04, HsSiO, H2SiOj", and the fields of dominance of H4Si04, HsSiO, and H2SiO as a function of pH.

The need for temperature cycling should be taken into account when designing or conducting tests. The nature of the test vessel should be considered for tests in aqueous solutions at temperatures above about 60°C since soluble constituents of the test vessel material can inhibit or accelerate the corrosion process. An inhibiting effect of soluble species from glass, notably silica, on the behaviour of steel in hot water has been shown . Pure quartz or polymeric materials are often more appropriate for test vessel construction. 

Quartz (Si02) and other silicates are generally stable in acidic solutions but will dissolve in highly alkaline waste solutions, decreasing the pH of the waste. The process by which this reaction occurs is complicated because it creates complex mixtures of nonionic and ionic species of silica. Scrivner and colleagues39 discuss these reactions in some detail. They observe that the silicates in solution buffer the liquid. Also, laboratory experiments in which alkaline wastes have been mixed... 

As a second example, we choose quartz (or any silica polymorph) as a component for a system containing an aqueous fluid and quartz. Now the mole number for the quartz component includes not only the silica in the quartz mineral, the real quartz, but the silica in solution in species such as SiC>2(aq) and IGSiO. Again, the mole numbers of component quartz and real quartz are not the same. A common mistake in geochemical modeling is confusing the components used to describe the composition of a system with the species and phases that are actually present. 

Mineral components are distributed among the mass of actual mineral in the system and the amount required to make up the dissolved species. In a system containing a mole of quartz, for example, there is (in the absence of other silica-bearing components) somewhat more than a mole of component quartz. The additional component mass is required to make up species such as Si02(aq) and IhSiO. Since vy moles of mineral component k go into making up each mole of secondary species j, mass balance is expressed as,... 

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

However, dissolution studies indicate that quartz does dissolve, albeit slowly, in vitro (Jurinski and Rimstidt, 2001) this suggests that organic species or some other ligand not considered in these calculations may be helping to complex aqueous silica and therefore driving the dissolution. Further work is needed to evaluate the potential role of organic complexes with silica in vivo. 

The nature of the silica-water interface is determined by adsorption/desorption of the species in the water. When a silicon oxide, e.g., quartz, is fractured, the initial surface is composed of dangling silicon and oxygen bonds (Fig. 4.30a) which are not stable and hydroxylate easily with available waterThe hydroxylated surface is dominated by SiOH groups (Fig. 4.30b). The initial adsorbed water adjacent to the surface is oriented and has properties different from the bulk water. As this adsorbed water layer increases to more than three monolayers, its properties become more like bulk water. The surface potential changes as a result of the adsorption of the ionic species in the water. °... 

Mineral composition, as determined by X-ray diffraction, shows a dominance of clay minerals, although quartz and opaline silica are persistent as sub-dominant and locally dominant or co-dominant (Table II). Of the clays, expandable lattice clay minerals, predominantly montmorillonite, occur in all the deposits with kaolinite or illite appearing as accessory or subdominant components. A marked contrast in the dominant clay species occurs between the brown oil shale unit and the two units below it at Condor. In these lower units, kaolinite is in greater abundance than other clays as well as quartz, an aspect already alluded to in the variations in Table I. (Loughnan (8) also noted that the structure of the kaolinite changes from ordered in the lower units to disordered in the brown oil shale unit). 

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